Modulation of antagonist binding to histamine H1-receptors by sodium ions and by 2-amino-2-hydroxymethyl-propan-1,3-diol HCl.

1. NaCl (100 mM) reduced the potency of (+)-N-methyl-4-methyldiphenhydramine ((+)-QMDP) as an inhibitor of the binding of [3H]-mepyramine to histamine H1-receptors on guinea-pig cerebellar membranes to a greater extent than that of mepyramine, consistent with the greater inhibitory effect of Na+ on the binding of [3H]-QMDP than on the binding of [3H]-mepyramine. 2. The concentration of 2-amino-2-hydroxymethyl-propan-1,3-diol HCl (Tris, HCl) buffer, pH 7.5, present had little effect on the temelastine-insensitive binding of [3H]-mepyramine, but caused a concentration-dependent inhibition of the binding of [3H]-mepyramine sensitive to 1 microM temelastine (H1-receptor binding), with an approximate IC50 of 75 mM, assuming that complete inhibition would have been achieved. 3. Inhibition of [3H]-mepyramine binding by Na+ was more marked in 10 mM than in 50 mM Tris HCl and was not evident in 200 mM Tris HCl. 4. The Kd for the temelastine-sensitive binding of [3H]-mepyramine measured in 10 mM Tris HCl, 0.24 +/- 0.01 nM, was increased by 2.2 +/- 0.2 fold by 100 mM NaCl, without any significant change in the maximum binding (Bmax). The Bmax for [3H]-mepyramine was similarly unchanged in 50 mM Tris HCl, but the Kd was increased 2.5 +/- 0.2 fold. 5. The Kd for the temelastine-sensitive binding of [3H]-mepyramine was also increased in 50 mM,compared with 10 mM, N-[2-hydroxyethyl]piperazine-N'-[2-ethanesulphonic acid] KOH (HEPES.KOH)buffer (Kd 0.25 +/- 0.02 nm in 10 mM HEPES), but the evidence for an interaction between HEPES and Na+ was less clear.6. The effect of 100 mM NaCl on the inhibition of [3H]-mepyramine binding in 10 mM Tris HCl was examined for a range of antagonists. The decrease in potency caused by Na+ was greatest for triprolidine, (+)-chlorpheniramine and benzilylcholine (9.6-10.3 fold increase in K1 values) but the binding of mepyramine and promethazine was much less affected (1.8 and 1.9 fold increase in Kd respectively). The Kd for temelastine was not significantly changed. In contrast to the general decrease in antagonist affinity in the presence of Na+, the for MDL 16,455A (4-[1-hydroxy-4-[4-(hydroxydiphenylmethyl)-1-piperidinyl]butyl]-alpha,alpha-dimethylbenzene acetic acid) was increased, but only by 1.5 fold.7. It is concluded that Na+ can act as an allosteric effector of the binding of antagonists at the histamine HI-receptor. Tris HCl also appears to have an allosteric action at the H1-receptor.